There's a few things that got me into the AT series (that's in a different topic I think).
Now that I know the series, what I get the most from it are:
- General setting (interesting on many levels). The setting and backstory development (over the different titles) are probably the biggest hook that keeps me interested in the series, and really looking forwards to the next game in the series. And the depth of the backstory and setting are awesomely detailed, thanks to the Settei books, giving a lot of material to think about, discuss, etc.
- Talk topics/dating sim/visual novel/cosmosphere elments. The whole helping cute girls fight their inner demons, talking to them, and getting closer to them over the course of the game as their protector (in battle, during dives, and through the rest of the story too), is pretty awesome. You can get pretty attached to the characters, and immersed in the story (wishing it was real, not wanting the game to end, etc), due to this. Probably one of the most disappointing things in the game is when you eventually run out of talk topics, and that you don't have much left to do besides finishing the game. Otoh: It's pretty awesome when just about every time you go to camp/inn, there's several more things to talk about with the girls.
- Synthesis/shop/event conversations. There are awesome, and really funny. I think this goes hand in hand with the character and personality design, and having some really skilled writers, who can play the characters and their personality quirks off against each other.
- Of course, the epic "save the
cheerleadergirls, save the world" storyline, where you have both some standard RPG elements, but also a lot of unexpected twists, most of which have a connection to the deeper backstory, and deeply involving the characters you're caring a lot about. When you're able to save them (and the world) from a terrible fate, it's a really great feeling of accomplishment.
- The innuendos are awesome too, though over the top sometimes. There's a lot of "did she really just say that?" moments, and many lols. What's also amusing is that the characters most of the time are saying these things innocently, and the writers let (or force, lol) the player to to hear an extra subtext, while really (as far as the characters are concerned) meaning the more innocent context. The AT series would actually be pretty perverse if the characters were really flirting with each other the way it sounds sometimes.
Well there's more, but this post is already tl;dr

PS: Ah, forgot that this was about Gust games in general, not specifically AT, lol. Basically I like the old-school style, and the music, plus the humor, quirky characters, and pretty interesting characters and settings. Plus the "save the girl, save the world" themes are great, too (mainly going by the Atelier Iris and AT series, have barely touched the other games so far).